Effect of rare earth elements on the microstructure and texture development in magnesium–manganese alloys during extrusion
Research highlights
▶ A new texture component is developed during extrusion of Mg–Mn alloys. ▶ Nd is a much stronger texture modifier compared to Ce or Y in Mg–Mn alloys. ▶ Recrystallisation is impeded in RE-containing Mg–Mn alloys. ▶ The weaker texture leads to increased ductility and decreased yield asymmetry.
Introduction
Extrusion plays an important role in extending the technical applications of wrought magnesium alloys. The broader use of such alloys and their semi-finished products is, however, restricted by their limited ductility and mechanical anisotropy at ambient temperatures [1]. These limitations result from the low symmetry of the hcp lattice structure of magnesium and its alloys which reduces the number of active slip systems. A further consequence of this is that strong textures develop during thermo-mechanical processing of magnesium alloys. Such textures result from preferred re-orientation due to the dominant deformation mechanisms such as basal slip and twinning. Furthermore, dynamic (DRX) or subsequent static (SRX) recrystallisation influences the texture development if processing is carried out at elevated temperatures [2]. During the extrusion of round bars, typically, a prismatic 〈10.0〉 fibre texture develops which may migrate to a 〈10.0〉–〈11.0〉 double fibre texture. This texture aligns the basal planes strongly into the extrusion direction [3], [4]. These preferentially developed grain orientations are unfavourable for basal slip, which is the most easily activated deformation mechanism and thus limit the ductility if tensile testing is carried out on samples oriented parallel to the extrusion direction.
It has recently been shown that sheets [5], [6], [7], [8] and extrusions [9], [10], [11] with weaker textures or broader orientation distributions of the basal planes have increased ductilities. The alloys used in these studies all contained a certain amount of a RE or a mixture which is usually based on cerium or neodymium (misch metal). Magnesium alloys containing Mn and single RE elements tend to form weak textures after extrusion. Profiles with such weak textures give rise to higher ductility as well as a small or vanishing asymmetry in tensile and compressive yield strength [12]. This present study aims to investigate the origin of such textures formed during extrusion and to analyse the recrystallisation behaviour. For this purpose, the microstructures and textures of extruded Mg–Mn based alloys containing single RE elements were examined using the XRD and EBSD techniques.
Section snippets
Experimental
In Table 1 the alloy compositions including the pure element additions of cerium (E), yttrium (W) and neodymium (N) are given. Alloy names follow ASTM procedures using the above letters for each element. All alloys were gravity cast to produce billets for extrusion which were machined to a diameter of 93 mm. The billets were homogenised at 350 °C for 15 h prior to extrusion. Indirect extrusion was carried out at 300 °C to produce round bars with a diameter of 17 mm, which corresponds to an extrusion
Microstructure
Micrographs from longitudinal sections of the extruded profiles are shown in Fig. 1. After slow extrusion, alloy M1 (Fig. 1a) exhibits a partially recrystallised microstructure which consists of large, elongated, unrecrystallised “grains” surrounded by newly formed, recrystallised grains [3]. After fast extrusion, the alloy shows a fully recrystallised microstructure and a significantly larger recrystallised grain size (average 70 μm) compared to slow extrusion (average 8 μm). The same basic
Microstructure and texture development
The results shown in Fig. 4, Fig. 5 have to be considered in the context of mechanisms that determine the development of microstructure and texture during processing. These are namely texture changes due to the active deformation mechanisms such as slip and twinning on the one hand, and in addition, changes that result from the nucleation and growth of new grains during DRX and SRX. The first aspect depends on the balance of the various deformation mechanisms by which the deformation texture is
Summary and conclusions
In summary, it was found that recrystallisation is impeded in RE-containing Mg–Mn alloys compared to alloy M1, which results in smaller grain sizes. This allows a new texture component to be developed during extrusion which implies changes in both the deformation and the recrystallisation mechanisms.
The texture of the profile is understood as a result of different competing mechanisms which lead to changes in the recrystallisation kinetics. It is likely that boundary pinning caused by the RE
Acknowledgements
The authors would like to thank Dr. P. A. Beaven at GKSS Research Centre for intensive discussions and Dr.-Ing. Sören Müller for his help with the extrusion experiments at the Extrusion Research Centre of the University of Technology in Berlin (Germany).
References (24)
- et al.
Scripta Mater.
(2005) - et al.
Acta Mater.
(2007) - et al.
J. Mater. Process. Technol.
(2010) - et al.
Scripta Metall. Mater.
(1994) - et al.
Scripta Mater.
(2008) - et al.
J. Mater. Process. Technol.
(2007) - et al.
Scripta Mater.
(2008) - et al.
Mater. Sci. Eng. A
(2008) - et al.
Scripta Mater.
(2010) - et al.
Acta Mater.
(2001)
Mater. Sci. Eng. A
Acta Mater.
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